I'm a total novice when it comes to biology, and I'm really just looking for an answer to a question that has been posed to me by people who deny evolution fervently.

Evolution dictates that we've gone from simple, single cell organisms to complex, multicellular organisms. If we consider the hand, for example, the hand is made up of various cells. Those cells are created according to DNA, which acts like a cell-level instruction manual.

Where are the "instructions" for where those cells go? I.e Where are the instructions that state that various cells have to line up in a thin, long, appendage structure to form a finger, or hand.

This isn't a particularly difficult question, I imagine. If it's been asked before, apologies. I don't know any of the technical terminology; this is just something that I would like to learn about.

I am afraid you would expect a simple answer but the subject is actually very broad. It is called Developmental biology or Ontogeny. This wikipedia article might give an introduction into this subject. Or maybe one wants to smake a summary this field. Or do you want specific details on the regulatory genes, signaling cascade, transcription factors, gradient of signaling molecule, mechanism of apoptosis and other things allowing to develop a hand?
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Remi.bOct 23 '13 at 15:45

Well, and I know this might be wildly ignorant, I was hoping for some sort of biological governing principle that will explain how cells behave in this way. If you could give me an answer to my question (even if it's complex, I'm pretty good at understanding stuff) that would be cool. I'm kind of just looking for an explanation of what happens to produce this effect (in this case, the hand).
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Chris CooneyOct 23 '13 at 15:52

Ok, I am not able to make this precise answer for the hand, but I'm sure someone will ;) I just wanted to give you the wikipedia links and to let you realize how this field is called and how big it is.
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Remi.bOct 23 '13 at 15:56

@ChrisCooney I would recommend the book "Endless forms most beautiful" by Sean B Carrol. I think it will give the best answer to your question in a simple, non technical language.
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biogirlOct 24 '13 at 10:15

1 Answer
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The main governing principle is very simple and one of the most elegant mechanisms in biology. It is based on concentration gradients.

Briefly, certain cells secrete signaling molecules (called morphogens) that are detected by other cells. These molecules then activate the transcription of genes in the target cells. Since the molecules in question are produced by a subset of cells, their concentration varies according to the distance between the receiving cell and the producing cells. Different concentrations of the signalling molecules have different effects and this allows the formation of really quite complex patterns.

So, different concentration thresholds of the morphogen result in different patterns of gene expression. This simple system can give extremely complex results (such as yourself, for example). It gets more complex when there are multiple morphogens acting in tandem. Say morphogen1 is produced by cells on the left and morphogen2 by cells on the right, cells in the middle will have varying concentrations of both morphogens and will react accordingly:

Just about every pattern formation during development boils down to differential concentrations of diffusing morphogen molecules, it really is a wonderful example of how simple systems can give rise to extremely complex results.

On an unrelated note, this has absolutely nothing to do with evolution. At least no more so than any other biological process. If your friends do not accept evolution, I don't see how they can use this as an argument. Even if a god created humanity in his or her image as these people tend to believe, babies are still born and they still develop in the womb. As far as I know, even the most fundamentalist Christians do not believe in a mechanic god that puts babies together in the womb from some kind of IKEA kit. Even if humans sprung fully formed from the hands of this god, individual infants are still formed out of multiple cells. There is therefore a need for a mechanism that can coordinate this process, whether this mechanism evolved or was designed is irrelevant, it is still there and extremely easily observable (images taken from here):

The image above shows the locations and concentrations of various morphogens in a fruit fly embryo. You can see the concentration gradients clearly.

Just to explicitly answer "Where are the instructions?", they are in the DNA of the cells (which tell them to secrete and respond to the morphogens that terdon speaks of). [And note terdon's point that this is just from studying cells as they exist here and now, and not related to how the cells got here.]
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mgkrebbsOct 23 '13 at 21:48

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Wow, thanks for the detailed answer. So, if I have understood this correctly; a single cell produces these Morphogens. Morphogens are chemicals that, depending on their concentration, cause varying behaviour in surrounding cells and causes the cells to change and form different patterns. Because of the huge variations in concentration, this allows for a huge range of patterns. Is it also these Morphogens that cause the cells to bind together in the way that forms "the hand"?
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Chris CooneyOct 24 '13 at 8:00

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@ChrisCooney Well, it's not a single cell that produces the morphogens but various groups of cells. You might want to look up Spemann's organiser for example. The rest of your analysis is pretty much correct, as for binding, that's a different issue but that too will be affected by morphogens, yes. It is the morphogens' concentration that causes one group of cells to form an index finger and another to a pinky for example.
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terdonOct 24 '13 at 16:21